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Not runtime polymorphic behavior

Just for the record, the above invocation of the method does not constitute runtime polymorphism (in my opinion) . I included that invocation of the method to serve as a backdrop for what follows.

If this is runtime polymorphism, it is not a very significant example, because there was no requirement for the runtime system to decide betweendifferent methods having the same name. The runtime system simply executed a method belonging to an object using a reference of the same type as the object.

This is runtime polymorphic behavior

However, the call to the method in Listing 4 does constitute runtime polymorphism.

Listing 4 . This is runtime polymorphic behavior.

//Following will also compile // and run due to polymorphic// behavior. ((A)var).m();

The statement in Listing 4 casts the reference down to type A and calls the method named m on that reference.

The method output

Here is the punch line. Not only does the statement in Listing 4 compile and run successfully, it produces the following output, (which is exactly the same output as before) :

m in class B

The same method was executed in both cases

Very important

It is very important to note that this output, (produced by casting the reference variable to type A instead of type B ) , is exactly the same as that produced by the earlier call to the method when thereference was cast to type B . This means that the same version of the method was executed in both cases.

This confirms that even though the type of the reference was converted to type A , (rather than type Object or type B ) , the overridden version of the method defined in class B was actually executed.

This is runtime polymorphic behavior in a nutshell.

The version of the method that was executed was based on the actual type of the object, B , and not on the type of the reference, A . This is an extremely powerful and useful concept.

Another invocation of the method

Now take a look at the statement in Listing 5 . Will this statement compile and execute successfully? If so, which version of the method will be executed?

Listing 5 . A failed attempt.

//Following will not compile //var.m();

(That was easy. The answer to the question is given in Listing 5 ) .

Compiler error

The code in Listing 5 attempts, unsuccessfully, to call the method named m using the reference variable named var , which is of type Object . The result is a compiler error, which reads something like the following:

error CS0117: 'object' does not contain a definition for 'm'

Some important rules

The Object class does not define a method named m . Therefore, the overridden method named m in the class named B is not an overridden version of a method that is defined in the class named Object .

Necessary, but not sufficient

Runtime polymorphism based on class inheritance requires that the type of the reference variable be a superclass of the class from which the object (on which the method will be called) is instantiated.

At least this requirement is true if a significant decision among methods is to be made.

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Source:  OpenStax, Xna game studio. OpenStax CNX. Feb 28, 2014 Download for free at https://legacy.cnx.org/content/col11634/1.6
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